Experimental Investigation under Unsteady Flow Conditionson Turbocharger Compressors for Automotive Gasoline Engines

Downsizing and turbocharging is today considered an effective way to get reduced CO2 emissions of automotive gasoline engines. An important aspect to take into account is the turbocharger behaviour under the strong unsteady flow conditions occurring in downsized engines intake and exhaust circuit. A better understanding of compressor and turbine pulsating flow performance can be accomplished through dedicated experimental investigations. This information can be a fundamental requisite to make simulation models able to accurately predict engine performance. Besides, the change of the compressor surge limit under pulsating flow conditions is another important phenomenon to be considered. In the paper the results of a broad experimental investigation performed on different turbocharger compressors matched to downsized gasoline engines are presented. Measurements were developed on the test facility operating at the University of Genoa, which allows investigations on automotive turbochargers both under steady and unsteady flow conditions. Tested turbocharger compressors were coupled to the current production engine intake circuit and the pulsating flow was generated by two different motor-driven cylinder heads fitted with a variable valve actuation system. Different levels of turbocharger rotational speed and pulse frequency were considered, also taking into account different intake valve opening strategies. For each operating condition compressor unsteady performance was evaluated starting from measurement of several instantaneous parameters (inlet and outlet static pressure, mass flow rate and turbocharger rotational speed). In the paper the influence of flow unsteadiness on compressor performance is analysed, referring to both 4-cylinder and 2-cylinder engine configuration.